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AJ exclusive: Moxon reveals designs for ‘sibling’ HS2 viaducts

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Moxon Architects has revealed its designs for two new viaducts for the High Speed 2 (HS2) rail line in the Chilterns Area of Outstanding Natural Beauty south of Aylesbury

HS2 said design work was progressing on both the 450m-long Wendover Dean viaduct and the 315m-long Small Dean viaduct 1.2km to the north, even though the outcome of a review into the future of the entire scheme has yet to be published.

In August, prime minister Boris Johnson ordered an independent review, led by Doug Oakervee, looking at the costs and benefits of HS2, ahead of a ‘go or no-go’ decision before the end of the year.

A spokesperson for HS2 said: ‘The government has been very clear to us that they don’t want to see any delay [to the construction programme] as a result of the Oakervee Review.

‘It’s business as usual. Design work is continuing, demolitions and site clearance are progressing and we’re hard at work setting up site compounds. The only thing delayed is ancient woodland clearance.’

The two viaducts have been conceived as related ‘sibling structures’ using a similar palette of materials, though the ‘form and detail of each has been developed with close consideration for its relationship with the wider landscape’.

Earlier, indicative designs for the two viaducts met with a mixed reaction, with some locals raising concerns about their impact on their surroundings and one branding the structures ‘grotesque’.

However, a spokesperson for the HS2 said they felt ‘cautiously optimistic’ about the almost-final designs which had recently gone out to public engagement and would soon be submitted for planning (under schedule 17 of the HS2 Act).

The HS2 spokesperson added that they thought local that people had recognised ‘how much effort had been put in’ on developing both schemes, especially in terms of the landscaping. The viaducts have also been narrowed since first being shown off in 2018.

Explaining the drivers behind the designs, Moxon’s managing director Ben Addy described the Wendover Dean bridge as ‘elegant and classically minimalist’, playing a ‘visual game of chiaroscuro’. The Small Dean viaduct, which cross two roads and a rail track, would be ‘more dramatic’ and feature a series of metallic vertebrae beneath the main deck, similar but grander in scale than the practice’s Somerstown bridge at King’s Cross.   

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Moxon Architects’ proposals for the Small Dean viaduct (October 2019)

Moxon Architects’ proposals for the Small Dean viaduct (October 2019)

He said: ‘Although they are high-profile – in that this is where the line breaks cover from the cuttings – they have been designed to be part of the landscape.

‘At Wendover, the focus [has been] less on the potential for a landmark-type structure that would risk dominating its surroundings and more on achieving a visually slender intervention’.

Although the exact cost of the viaducts has not been revealed, Addy said: ‘[Some kind of viaduct] could certainly have been built for less money than these are going to cost. There’s no question of that. But the absolute priority has been on [coming up with a design] that respects the landscape.’

In terms of the environmental impact, the HS2 team insists the since the project’s inception the carbon costs of the entire rail scheme had, through clever engineering and designed, been halved.

Addy defended the amount of concrete that would still be necessary in the viaducts. He said: ‘As well as the arguments about the carbon in use saving [through electric rail travel] we have also considered the embodied carbon.

’All HS2 structures have been designed with a 120-year design life between major maintenance works. That blows anything else out of the water – most of the things we do have a 25-year design to major maintenance life. These have been designed to be incredibly robust’.

He added: ‘The total CO2 emitted during the construction of HS2’s phase 1 is estimated to be 6 million tonnes – this compares favourably with the 11 million tonnes released in the UK by a single month of road transport. Which puts things into perspective.’       

Depending on the findings from the Oakerveee Review, construction work on the viaducts could start in 2021 and complete in 2024.

The architect’s view

Wendover Dean Viaduct

The proposals have been developed in accordance with the HS2 Design Vision and the Chilterns AONB detailed design principles. The appearance of the viaduct in relation to the preservation of the local environment has been carefully considered and responded to, in particular with respect to views of the structure in the wider context and the pre-eminence of existing landscape characteristics and topography.

The intention to maintain visual continuity of the landscape surrounding the Wendover Dean Viaduct is as critical to the design response as the form and detail of the structure itself. Landscape continuity beneath the Viaduct will be provided through the repair and reinstatement of historic field boundaries and hedgerows – the ancient field pattern and the day to day farming activity that it supports must continue to ‘flow’ uninterrupted beneath the static simplicity of the Viaduct structure.

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Wdv update 01

The viaduct design follows the classic viaduct principles of simple, evenly spaced vertical piers combined with predominantly below deck structure. Arranged in a carefully composed relationship of span depth and support spacing that balances the silhouette of the structure against a near symmetrical ground profile, the proposal also maximises the air draught at the end spans where the bridge comes closest to ground level. In cross section, the deck design has been developed to privilege the outermost edge of the structure – the parapets – by providing as slender a cantilever from the central spine as possible while presenting a bluff face to the main girders, throwing the majority of the deck structure into shadow and visually emphasising the relatively thin leading edge of the deck. This deliberate interplay of light and dark is further emphasised through the distinctive pier form and the contrasting use of high-quality concrete for the parapet and dark weathering steel for the girders: the parapets are sloped to catch the sun and sky light while the girders will weather over time to the characteristic dark russet finish of exposed weathering steel.

When viewed from a distance the matt finish of the steel will appear similar in tone to the background landscape – picking out the edge of the parapet as a bright, thin horizontal band ‘hovering’ over slender piers. To emphasise this effect the Viaduct piers are extended beyond the bearing shelf to almost connect with the parapet – further recessing the main beams and accentuating the outer faces of the structure.

Small Dean Viaduct

With the deck soffit just 6m above the road the design of Small Dean viaduct will be experienced at especially close quarters, making the detailed design of the structure crucial to its success.

The skew alignment at Small Dean makes the pier design particularly important – the transverse ‘face’ of the piers will be visually prominent, being presented at either side of the carriageway to anyone travelling along the A413 London Road. The level of detail and a consistent, legible, architectural language is therefore of special importance: the form, detail and finish will come under particular scrutiny in this location. In the case of both the Wendover Viaducts the formal expression of the piers takes on anthropomorphic quality - arms and hands holding the bridge deck in a ‘Y’ form – a legible expression of structural forces coming down through the widely spaced bearings and into the slender stem of the pier column. The pier form also allows for the placement of supports at a closer proximity to the A413 – the hammerhead is located at an elevated height relative to the clearance envelope of the carriageway and this contributes to the even spacing of the supports.

The Small Dean viaduct will be experienced at especially close quarters

The formal characteristics of taut edges and soft radial transitions in the pier design cascade to the design of the parapets, deck stiffeners, noise barriers and viaduct abutments. Unifying details will appear at all scales in the design with patterns in the pier recesses intended to be carried through in design development to reliefs on the parapet and viaduct soffit.

Where the weathering steel at Wendover Dean will serve to relate the deck structure to the landscape at Small Dean the dark muted finish will harmonise with foliage, losing the structure amongst the trees and increasing the relative prominence of the white concrete piers and parapet edges.

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Sdv idp winter2 08

Project data

Location Wendover, Buckinghamshire
Local authority Aylesbury Vale District Council
Type of project High Speed rail/infrastructure
Client Eiffage Kier JV (Area Central Main works Civils Contractors) for HS2 Ltd
Architect Moxon Architects
Landscape architect COWI-Arcadis-Setec JV (and Moxon Architects on the Small Dean viaduct)
Planning consultant COWI-Arcadis-Setec JV
Structural engineer COWI-Arcadis-Setec JV
M&E consultant COWI-Arcadis-Setec JV
Quantity surveyor Eiffage Kier JV
CDM adviser COWI-Arcadis-Setec JV
Lighting consultant Moxon/COWI-Arcadis-Setec JV
Main contractor Eiffage Kier
Funding Public
Start on site date 2021
Completion date 2024 (start and end dates subject to Oakervee review)
Height and length: Wendover Dean viaduct: length 450m; overall max height above ground level circa 20m - Small Dean viaduct: length 315m; overall max height above carriageway 12.5m

  • 3 Comments

Readers' comments (3)

  • I wouldn't be very happy with this design if I was a local. This is an area where HS2 is detested, and the surrounding landscape loved. The design seems to be deliberately bold, and the sight lines are unnecessarily thick, serving only to antagonise the locals.

    From a structural perspective, I do not understand why the parapets have not been integrated into the sectional height of the structure, to reduce the overall thickness of the sight lines. The colour of the steel is good and understated: it doesn't need the glittering parapet to show off - the sky and landscape should be doing that.

    It is too showy, and too French Brutalist to be welcomed here.

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  • As Moxon suggest, these are rather beautiful designs.

    But I would contend that an architect's responsibility extends further than their just structure considered in isolation.

    Attractive as these viaducts are it is entirely inappropriate to run them through the still more beautiful Chiltern countryside.

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  • Judging by the design of this viaduct, it seems HS2 are happy to deprive passengers on HS2 trains of having any views of passing landscapes due to the tall opaque parapets on the viaduct deck. This could deter passengers from choosing to travel on HS2 as opposed to classic lines which have clear landscape views.

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